Never happened to me, but I can absolutely imagine how it could happen, especially in this woman's situation. If you think there is no way you can get pregnant and you were expecting your periods to stop anyway, you'd probably assume a little weight gain (some don't put much on - how, how???) was down to overeating or middle age spread. And if you don't know you're pregnant, labour pains could possibly be put down to really bad stomach cramps.
And Stranded, what's with the profanity that's just appeared in the middle of your name ?